What Is The Cause And Treatment For Frequent Urinary Tract Infections?

Hi, My father 69 years old experiencing frequent UTIs. First occurred in Jan 16 severe fever got admitted and monocef injection was use to treat the infection he recovered in 3-4 days. Then again in March end 16, this time Urine culture test was performed and found growth of E.coli and Gentamicin inj. was recommended was given 10 days morning and evening though he recovered again in first 3-4 days. Again a UTI occurred in 10th May 16 this time amikacin inj was given for 7 days through veins. After around 5-7 days he recovered.Just now we did a urine culture again and found a growth of E.coli again though he is quite well and not experiencing any burn or pain. What should we do? He is diabetic and had a mild heart attack 6 years back no blockage was found.